Chapter 12 - 1 Chemistry 1411 - Dr Lyon Gas Laws 12-2 Some...

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1 Chemistry 1411 --- Dr Lyon Gas Laws 12-2 Some Common Properties of Gases: 1. Gases can be compressed into smaller volumes ; that is, their densities can be increased by applying increased pressure . 2. Gases exert pressure on their surroundings ; in turn, pressure must be exerted to confine gases. 3. Gases expand without limits , and so gas samples completely and uniformly occupy the volume of any container. 4. Gases diffuse into one another , and so samples of gas placed in the same container mix completely. Conversely, different gases in a mixture do not separate on standing. 5. The amounts and properties of gases are described in terms of temperature, pressure, the volume occupied, and the number of molecules present . For example, a sample of gas occupies a greater volume when hot than it does when cold at the same pressure, but the number of molecules does not change. 12-3 Pressure (force per unit area) Barometers are used to measure the pressure the atmospheric pressure (atm). Figure 12-1, page 437. Because mercury barometers are very common: gas pressure is often expressed in mmHg gas pressure is also expressed in torr 1 mmHg = 1 torr atmospheric pressure decreases with increasing elevation (less mass of air) At 0 o C, 1.0 atm = 760 mmHg = 760 torr The SI unit is the pascal (Pa): 1.0 atm = 1.013 x 10 5 Pa Learn these units of pressure!!!
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2 Boyle’s Law : The effect of pressure on volume of gas. At constant temperature and with a constant number of moles gas: PV = k Where: P = pressure V = volume k = a constant (determined experimentally) Robert Boyle (seventeenth century) showed that the volume of a sample of gas, under constant temperature, would decrease when the pressure was increased. An inverse relationship exists between pressure and volume When the pressure and volume were multiplied, regardless of the pressure change, the product was a certain constant number (k). From the data and graphs shown on the top of page 442, Boyle determined that ideal gases (those at normal temperatures and pressures) have the following relationship: therefore, P 1 V 1 = P 2 V 2 P 1 V 1 = k P 2 V 2 = k Charles’ Law : The effect of absolute temperature on the volume of gases. At constant pressure and with a constant number of moles of gas: V = kT Where: T = absolute temperature V = volume k = a constant (determined experimentally) Jacques Charles and Joseph Gay-Lussac (nineteenth century) showed that the volume of a sample of gas, under constant pressure, would increase when the temperature was increased. A direct relationship exists between absolute temperature and volume When the volume is divided by the absolute temperature, regardless of the temperature change, the product was a certain constant number (k). From the data and graphs, (Figure 12-5, top of page 445) Charles determined that ideal gases (those at normal temperatures and pressures) have the following relationship: V 1 = k therefore, V 1 = V 2 T 1 T 2 T 1 V 2 = k T 2
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3 From the graph, absolute zero (-273 o C) was extrapolated as a result of the measurements of a gas (at different pressures) at increasingly lower temperatures.
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Chapter 12 - 1 Chemistry 1411 - Dr Lyon Gas Laws 12-2 Some...

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